1. Field of the Invention
This invention relates to a low profile lift assembly having a horizontal support platform that is movable toward and away from a base in parallel relationship thereto. The assembly has unique extension structure which significantly decreases the ratio of collapsed height to extended height, maximizes the permissible load on the operating components and reduces the amount of force necessary to overcome resistance to movement when the assembly is extended from its initially fully collapsed state. The portable work stand thereby presented is especially useful for supporting a mechanic in a horizontal position or for carrying a load such as the engine of an aircraft or an automobile, and includes a number of linkages which provide substantially equal support over the entire area of the work platform regardless of the degree of extension of the latter from the base.
2. Description of the Prior Art
Aircraft mechanics and similar workers have long felt a need for a roller creeper having a platform which can be extended upwardly to enable the user to more conveniently work on the underside of an aircraft which is normally substantially higher from the ground than the underside of an automobile or truck. Conventional practice of mechanics has been to raise the effective height of typical creepers in incremental units by adding books, boxes, boards or other make-shift members to the top of the creeper until a proper elevation is approximately attained. As can be appreciated, such practice is inconvenient at best and usually cannot provide support for the worker at just the right height in steady fashion and free of risk.
However, comfort of the mechanic is an often overlooked factor which can greatly contribute to work productivity in terms of both the time necessary to complete the job as well as the overall quality of the work once finished. A mechanic working on the underside of aircraft without proper support at a comfortable working distance from the aircraft components constantly shifts his or her body to relieve muscle stress and fatigue, and these body movements, whether consciously or unconsciously initiated, substantially increase the time necessary to complete the job and also contribute somewhat to mental stress since the normal train of thought is consequently interrupted.
In other instances, it would be desirable to have a wheeled dolly available which is provided with a vertically extensible support platform for carrying tools, engines or other items and that is selectively movable both in vertical as well as in horizontal directions. For example, a dolly for removing engines from vehicles should be lightweight and thus readily portable, while having sufficient strength to safely carry the engine at a height convenient for performing work functions thereon, rendering unnecessary the practice of transferring the engine from the dolly to a work table once the engine is removed from the vehicle. As another example, it would be desirable to be able to remove engines from relatively large helicopters by provision of a portable wheeled dolly which can be rolled up a ramp into the fuselage, and wherein a work supporting surface can be raised a sufficient height in order to meet the engine as it is uncoupled from its normal elevated position with respect to the helicopter deck.
Unfortunately, conventional scissors lift tables are unsatisfactory for a number of reasons. Scissors lifts often comprise two pairs of elongated, rigid links, one pair of which is pivotally connected to the base and shiftably connected to the platform, and the other pair of which is pivotally connected to the platform and shiftably connected to the base. A hydraulic piston and cylinder assembly secured to the base and coupled to the linkages is operable such that extension or retraction of the piston shifts the platform either away from or toward the base. However, as the linkages in the conventional scissors lift table are pivoted to raise the support platform, adjacent ends of the linkages simultaneously shift toward one another. The result is that the raised platform is no longer supported equally in all corners by the linkages; one end of the platform in effect "overhangs" the linkages. To overcome this deficiency, scissors lift tables are often manufactured of relatively heavy materials and are derated in capacity so that loads carried by the unsupported, or cantilevered region of the platform cannot tip over the assembly. As can be appreciated, such scissors lift tables are consequently heavy and cannot be readily transported by a single person.
In the construction of lift tables having linkages which are swingable in arcs to shift a work supporting platform toward and away from a base, it is difficult to provide a sufficient amount of "starting" force for initially separating the platform from the base especially when the platform is carrying a load. For the most part, prior art lift tables have overcome starting forces by provision of large piston and cylinder assemblies, and have utilized relatively hefty linkages, pins and pivot assemblies to minimize bending and excessive stress. Again, however, such practices unacceptably increase the weight of the entire assembly to the point where the latter cannot be conveniently carried by the worker when necessary while still remaining a load bearing capability sufficient to meet the demands of the job.